The team analysed blood samples from 1400 volunteers enrolled in a decades-long epidemiological study. At the start of the study, the researchers detected antibodies to Ad-36 in 14.5 per cent of the subjects – a prevalence in line with studies on the US adult population.

Ten years later, those individuals naturally infected with Ad-36 had a higher body mass index and body fat percentage than those who were not infected – but their blood sugar and insulin levels were healthier.

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Animal and cell studies offer an explanation, says Dhurandhar. They suggest that Ad-36 increases the number and size of fat cells, or adipocytes, providing additional “depots” for any fat coming from excessive calorie consumption.

“That is where the apparent paradox is not really a paradox,” says Nicola Abate, an endocrinologist at the University of Texas Medical Branch at Galveston, who was not involved in the study. By creating extra fat storage, Ad-36 prevents fat from travelling to other areas like the liver, where it can have toxic effects. The adipocytes may also store more sugar, helping to keep blood sugar levels under control.

Abate says the findings underscore the need to separate weight from metabolic disease&colon; “It’s not how much fat a person has that determines the health complications, it’s the function of adipose tissue.”

Dhurandhar presented the work at the Obesity Society meeting in San Antonio, Texas, last month. He says it may lead to a potential therapy for type 2 diabetes.